GB2295896A - Calibration of flowmeters - Google Patents

Abstract

A flowmeter 2 in a line delivering fluid from a tank 1 to a storage tank or road/rail vehicle 6 via a valve 3 and which includes a first pulse generator producing a pulse stream 7 related to the flow is connected, for purposes of calibration, in series with a pre-calibrated reference flowmeter 4 having a second pulse generator producing a second pulse stream 8. One of the pulse streams, e.g. the stream 7 of the test meter 2, is a coarse pulse stream, while the other is a high resolution pulse stream. The two streams are compared, e.g. in a computer 9, and the results of several comparisons are used to establish a calibration adjustment of the flowmeter under test. The comparisons may be of consecutive or overlapping pulse streams (Fig. 2); the computer may produce a test certificate. Apparatus suitable for use in the method of the invention is described; the fluid delivery system may be a gantry system; the calibration apparatus may be mounted on a vehicle. <IMAGE>

Description

CALIBRATION OF FLOWMETERS The invention relates to a method and apparatus for calibrating or proving flowmeters.

Flowmeters are commonly used in industry to determine the volume of liquid transferred from one place to another. In particular they are used in gantry installations to measure the volume of liquids dispensed into road or rail tankers.

It is important that such flowmeters are correctly calibrated to ensure that the fluid dispensed is accurately measured.

Flowmeters are conventionally calibrated by connecting a pre-calibrated reference meter in series with the meter under test. The working fluid is passed out through both meters, the meters usually being connected to one another via hoses.

Conventionally a hand operated valve, supplied as part of the reference metering equipment, is used to start the flow of the fluid and to stop it after a pre-determined time or after a predetermined quantity has passed through the meters. The volume of fluid registered by the reference meter, either on a mechanical counter or on an electrical counter, is then compared with that registered by the meter under test. The performance of the reference meter has been established by prior calibration and the volume of fluid registered by the reference meter can be corrected to account for operating conditions, such as temperature and pressure, through the use of previously established correction factors.

It is normal to carry out at least three runs at a given flow rate, and provided the results of these runs meet certain performance criteria, such as repeatability, then the results are regarded as acceptable. The results can then be used to adjust the meter under test.

Having to repeat the runs through the meters is time consuming and leads to a period when the meter cannot be used to dispense the fluids.

Fluctuations in the flow rate and conditions can also occur because, for example, the hand operated valve may not be opened to the same extent as on previous runs, temperature variations due to the heating of the fluid by sunlight in pipelines as opposed to storage tanks, and due to pumping capacity limitations when a second tanker is loading on the same system.

East German patent application number DD127661A (Schmidt) provides an electrical measuring system based on the above method. The meter under test and the calibrated reference meter are each connected to pulse generators. The pulses produced are proportional to the rate of flow of fluid through the meters. The pulses from the pulse generators are measured by electronic circuitry which converts the pulses into a voltage at a precision capacitor, the voltage being proportional to the number of pulses received.

The disadvantage of this system is that several runs are required to achieve the desired standard of accuracy.

An aim of the current invention is to provide a method of calibrating a flowmeter with increased accuracy and without the need of repeated calibration runs. Sufficient data may be gathered during one continuous run without the need to stop and restart the flow between successive runs.

Viewed from one aspect the invention provides a method of calibrating a flowmeter comprising arranging a flowmeter with a first pulse generator in series with a pre-calibrated reference flowmeter with a second pulse generator, wherein; one of the pulse generators generates a coarse pulse stream which is proportional to the reading from its respective flowmeter; the other pulse generator generates a high resolution pulse stream which is proportional to the reading from its respective f lowmeter; the method further comprising comparing the output of the two pulse streams to establish a calibration adjustment of the flowmeter under test.

The coarse pulse stream may be at a lower frequency than the high resolution pulse stream.

The pulse outputs from the two meters may be fed into a trigger/counter unit or directly into a computer, both of which must be intrinsically safe or suitably explosion-proof if used in a hazardous area.

Preferably a predetermined number of pulses from the coarse stream is counted before the flow of fluid is shut off by a valve. During this time the pulses generated may be recorded. Preferably the number of pulses from the coarse stream is calculated so that the number of pulses from the high resolution stream is high enough to produce a statistically accurate calculation of the variance between the flow rate measured by the flowmeter under test and the pre-calibrated reference flowmeter. The number of high frequency pulses received may be at least 5000.

Data processing may involve dividing the total pulse count into either consecutive groups or overlapping groups of smaller numbers of pulses, which may be divided on the basis of the count from the coarse pulse generator. In effect therefore, during the single run when pulses are being collected from both meters, the system analyzes the data as a group of, for example, five or ten, shorter runs. The data is analyzed statistically or using a simple repeatability criterion to establish the acceptability of the results.

The use of the two pulse streams of different frequency enables any variance during the recording of the data to be identified and reduces the chances of errors in the calibration system used.

The average resulting from the short runs is used as the basis for establishing the calibration factor or the calibration adjustment required for the meter under test. The adjustment is then made electronically or mechanically to the meter under test.

The method of the invention may be applied to turbine or positive displacement (pd) meters, in the case of pd meters it may be used whether the meter is fitted with a mechanical counter or an electronic display unit.

The coarse pulse stream may be derived from the existing pulse stream from the pulse generator of a positive displacement meter or the pick-up of a turbine meter. Alternatively, in the case of a positive displacement meter, a pulse generator or shaft encoder can be installed on the output shaft of the meter under test using a special adaptor.

This installation must allow the existing pulse generator or mechanical counter to continue to be driven in the normal way during the meter calibration operation.

The pre-calibrated reference flowmeter may have a high resolution pulse generator attached to its output shaft. Preferably it is a positive displacement meter.

Alternatively the flowmeter under test may have the high resolution pulse generator, and the coarse pulse stream would then be derived from the reference flowmeter.

Viewed from another aspect the invention provides apparatus suitable for use in the method of the invention.

Preferably the apparatus is for use with a gantry fluid delivery system or similar fluid measurement system.

With a suitable computer connected on-line to the system all the data processing and analysis, and all the corrections for temperature, pressure etc.

to be applied to the reference meter can be carried out automatically. Temperature and pressure data may be read automatically by the system from transmitters mounted on the reference meter. Alternatively this data may be entered manually by the operator.

A number of calibration runs may be used across the full spectrum of the conditions, such as flow rates and pressure, used by the flowmeter under test. These can be stored by the computer which can then produce a test certificate if required.

An embodiment of the invention will now be described by way of example with reference to the following drawings: Figure 1 is a schematic diagram of apparatus of the current invention; and Figure 2 is a schematic representation of the pulse streams and how they are separated into groups of pulses.

Referring firstly to Figure 1, gantry fluid delivery systems conventionally comprise a storage tank (1) connected via a meter (2) to a valve (3) which controls the movement of fluid into the road or rail tanker.

The calibration system of the invention may suitably be on a vehicle for easy transportation and comprises a pre-calibrated reference flowmeter (4) which is connected via means of suitable hosing to the valve (3) of the fluid delivery system. The outflow from the pre-calibrated reference flowmeter (4) may be suitably connected via a valve (5) to a storage tank or a road rail tanker (6).

In use the valve (3) may be fully opened, and the flow of fluid through the system may be controlled by the second valve (5).

A pulse stream (7, 8) is generated by a pulse generator integral to each flowmeter (2, 4) the frequency of which is proportional to the reading from the flowmeters (2, 4).

The pulse generator of the flowmeter (2) under test produces a coarse pulse stream, and the pulse generator of the reference flowmeter (4) produces a higher frequency, high resolution pulse stream as previously described.

Figure 2(a) shows a schematic representation of the coarse pulse stream and Figure 2(b) shows a schematic representation of the high resolution pulse stream.

The pulse streams (7, 8) are fed directly into a computer (9). After a predetermined number of pulses of the coarse pulse streams (7), valve (5) may be automatically turned off by a signal from the computer (9). Alternatively, valve (5) may be turned off manually after sufficient fluid has passed through the meters.

The data from the two sets of pulse streams may be separated into consecutive groups (Figure 2(c)) or overlapping groups of pulses (Figure 2(d)). The data between the groups is then compared to look for any variance during the recording of the data.

The average from the pulses in each pulse stream is then compared with the other pulse stream and used as a basis for establishing the calibration factor required for the flowmeter under test.

The calibration may then be repeated under different pressures or flow rates to calibrate the flowmeter under its full working range.

In an alternative embodiment the pulse generator of either flowmeter may not be integrally provided but may comprise a separate unit attached to the respective flowmeter.

In another alternative embodiment the pulse generator of the reference flowmeter (4) may produce the coarse pulse stream and the pulse generator of the flowmeter (2) under test may produce the high resolution pulse stream.

In a further alternative embodiment, the pulse streams (7, 8) may conveniently be fed into a trigger/counter unit prior to the computer (9).

Claims (10)

1. A method of calibrating a flowmeter cpmprising arranging a flowmeter with a first pulse generator in series with a pre-calibrated reference flowmeter with a second pulse generator, wherein: - one of the pulse generators generates a coarse pulse stream which is proportional to a reading from its respective flowmeter; - the other pulse generator generates a high resolution pulse stream which is proportional to the reading from its respective flowmeter; and - comparing the output of the two pulse streams to establish a calibration adjustment of the flowmeter under test.

2. A method according to Claim 2, wherein the coarse pulse stream is at a lower frequency than the high resolution pulse stream.

3. A method according to Claim 2, wherein the output from the two pulse streams is directed into a trigger/counter unit or a computer.

4. A method according to any previous claim, wherein a predetermined number of pulses from the coarse pulse stream is counted before the flow of liquid through the flowmeters is shut off.

5. A method according to Claim 4, wherein the predetermined number of pulses is calculated so that the number of pulses from the high resolution pulse stream is high enough to produce a statistically accurate calculation of the variance between the flow rate measure by the flow rate under test and the pre-calibrated reference f lowmeter.

6. Apparatus for use with the method according to any previous claim.

7. Apparatus according to Claim 6, comprising a gantry fluid delivery system.

8. Apparatus according to Claims 6 or 7, wherein the flowmeters are turbine or positive displacement meters.

9. Method of calibrating a flowmeter substantially as hereinbefore described with reference to the description and/or accompanying diagrams.

10. Apparatus for a method of calibrating a flowmeter, substantially as hereinbefore described with reference to the description and/or accompanying diagrams.